Tetra Tech, Inc.

10306 Eaton Place, Suite 340

Fairfax, VA 22030

phone 703-385-6000

fax 703-385-6007

DATE: December 8, 2008

TO: Paul Shriner, EPA

FROM: Kelly Meadows,

SUBJECT: Analysis of swim speed data

Tetra Tech was tasked with researching 

0.5 feet per second (fps) threshold for fish swim speed in 

impingement mortality in the 3

treatment of swim speed in the previous rules, as well as provide 

the development of the revised 

Background

Impingement is generally correlated to three factors:

swim speed.1 The latter two factors are closely related, as the ability of fish to evade 

impingement depends on the swimming ability of the individual fish and the intake 

velocity against which it is attempting to e

regulating the intake velocity at cooling water intake structures 

way to minimize impingement impacts.

Where to Measure Intake Velocity

Intake velocity can be measured at a number of loc

significant impact on the effectiveness of the 

 Far-field: Measured at a large distance from the intake structure. T

area of effect of an intake structure dissipates over a relatively small 

become lost in ambient currents in the waterbody

be a useful metric for reducing impingement.

 

1

See 69 FR 41612. In contrast, entrainment is generally related only

MEMORANDUM

10306 Eaton Place, Suite 340

December 8, 2008

Paul Shriner, EPA

Kelly Meadows, Tetra Tech

Analysis of swim speed data

Tetra Tech was tasked with researching the origins of (and continued applicability of) the 

0.5 feet per second (fps) threshold for fish swim speed in reducing impingement and 

the 316(b) regulations. The sections below discuss the 

in the previous rules, as well as provide discussion to consider in 

revised Phase II rule.

Impingement is generally correlated to three factors: intake flow, intake velocity, and fish 

The latter two factors are closely related, as the ability of fish to evade 

impingement depends on the swimming ability of the individual fish and the intake 

velocity against which it is attempting to escape. EPA has consistently recognized that 

regulating the intake velocity at cooling water intake structures (CWIS) is an effective 

way to minimize impingement impacts.

Where to Measure Intake Velocity

Intake velocity can be measured at a number of locations, each of which can have a 

significant impact on the effectiveness of the velocity threshold:

Measured at a large distance from the intake structure. Typically, the 

effect of an intake structure dissipates over a relatively small distance

become lost in ambient currents in the waterbody. Far-field velocity is unlikely to 

be a useful metric for reducing impingement.

 

See 69 FR 41612. In contrast, entrainment is generally related only to intake flow.

1

the origins of (and continued applicability of) the 

reducing impingement and 

The sections below discuss the 

to consider in 

intake flow, intake velocity, and fish 

The latter two factors are closely related, as the ability of fish to evade 

impingement depends on the swimming ability of the individual fish and the intake 

scape. EPA has consistently recognized that 

is an effective 

ations, each of which can have a 

ypically, the 

distance, as it 

field velocity is unlikely to 
2

 Approach velocity: Measured at some distance in front of the screen (often 

defined as a few inches [EPRI 2000]), just at or before the point where velocities 

increase before passing through the screen.

 Through-screen: Measured in the open area of the screen, as water and organisms 

pass through. Through-screen velocity has been EPA’s preferred metric for the 

Phase I, II, and III rulemakings.

Intake velocity measurements should be measured at the “first point of contact,” where an 

organisms would be presented with the first (and perhaps best) opportunity to detect the 

intake structure and avoid it. For example, a facility that employs an intake canal should 

measure the intake velocity at the mouth of the canal, as organisms can detect the intake 

and avoid the canal. If the intake velocity was measured at the intake screen (and not the 

mouth of the canal), organisms may be drawn into the canal but be unable to return to the 

primary waterbody and may eventually be impinged.

Adding numbers to this example, a facility with an intake canal has an intake velocity of 

1.0 fps at the mouth of the canal and 0.5 fps at its traveling screens near the facility. In 

this case, organisms are not provided with the optimal opportunity to avoid being 

impinged, as they may be drawn into the canal due to the higher intake velocity. Once in 

the canal, escape may be difficult and the organism may eventually be impinged, despite 

the lower intake velocity at the screen itself.

Another example would be a facility with an offshore intake structure that supplies 

cooling water to an onshore forebay. The intake velocity should be measured at the 

offshore intake structure, as an organism drawn into the CWIS and forebay would have 

no chance of escape (except possibly through a fish return; however, survival rates for 

organisms passing through a fish return is often low).

Similarly, measuring the velocity of flow in the conduits behind the screen is not 

appropriate, as any organism behind the screen will not have an opportunity to escape.2

Other Biological Factors Affecting the Swim Speed Threshold

Two other biological factors can also affect the ability of fish to escape an intake 

structure. EPA’s approach to account for these factors is also discussed.

 Horizontal versus vertical flow vectors: Fish are generally unable to detect 

changes in flow or velocity in a vertical direction. Any intake that withdraws 

water vertically (e.g., an uncapped offshore intake) poses a significant 

impingement hazard.

o All intake velocity measurements should be measured in the horizontal 

direction.

 Categories of swim speed: Fish typically exhibit two general types of 

swimming—prolonged continuous swimming or burst swimming (EPRI 2000). 

The former is the most common and is slower. Burst swimming is usually 

 

2

In fact, some intake systems are designed to have high (>3-5 fps) velocities behind the screens, as this can 

inhibit the colonization of fouling organisms.
3

induced by an outside stimulus (e.g., chasing prey, escaping a disturbance) and is 

much higher, but typically the organisms only travel a short distance.

o Burst swim speeds are higher and may provide fish with an improved 

ability to react to stimuli (including intake screens). However, even with 

short periods of burst swimming, a fish may avoid initial contact with the 

screen but may be unable to eventually escape the area of the intake 

structure. As such, EPA’s conservative approach of using swim 

continuous/prolonged swimming speeds in its analysis is appropriate.

Phase I Rule

The Phase I rule established requirements for all new intakes to have a maximum design 

intake velocity of 0.5 fps.3 The final rule describes the analysis EPA used: precedent 

from previous Federal studies, existing data, and an analysis of fish length versus swim 

speed. (See 66 FR 65274.) The information collected indicated that a 0.5 fps threshold 

would be protective of 96% of the species and life stages found in the data.

In addition, the Electric Power Research Institute (EPRI) submitted a 2000 study with its 

comments on the Phase I Notice of Data Availability (NODA) titled “Technical 

evaluation of the utility of intake approach velocity as an indicator of potential adverse 

environmental impact under Clean Water Act Section 316(b).” This comprehensive 

review of swim speed literature contained most of the data used by EPA in justifying the 

0.5 fps threshold. EPRI agreed that intake velocity was an appropriate regulatory

criterion, and that while it preferred a site-specific approach to setting intake velocity 

limits, that a limit of 0.5 fps was a useful threshold for screening out significant 

impingement events at CWISs.

In the NODA and in a memo to the record,4EPA discussed the intake velocities at 

existing facilities, noting that most facilities constructed in the last 15 years already meet 

the 0.5 fps threshold.

EPA’s intake velocity threshold was challenged in litigation but was upheld by the 

Second Circuit.

Phase II and Phase III Rules

The 2004 Phase II rule used the same data, analyses and conclusions presented in Phase I 

to support a compliance alternative where an intake at a facility with a design throughscreen velocity of 0.5 fps meets the impingement requirements. Similarly, the proposed 

Phase III rule utilized the same regulatory framework as the Phase II rule, including the 

0.5 fps intake velocity threshold. In the final Phase III rule, EPA opted not to regulate 

land-based facilities, but continued to impose the intake velocity requirements on certain 

offshore facilities.5

 

3

See 128.84(b)(2) and (c)(1).

4

See 66 FR 28864 and DCN 2-030, respectively.

5

See 125.134(b)(2).
4

Revised Phase II Rule

In response to the Second Circuit decision, EPA is revising the Phase II rule. As part of 

this rulemaking, EPA opted to briefly re-examine the basis for the 0.5 fps threshold, as 

well as conduct additional screening analyses.

In reviewing the swim speed data in the record, the previous conclusions continue to be 

supported by the data. 

First, many intakes already meet this standard, thereby reducing the burden of meeting 

the requirement.6 In fact, many intake technologies installed today (e.g., cylindrical 

wedgewire screens) are specifically designed to meet the 0.5 fps threshold.

Second, as described above, a 0.5 ft/sec through-screen velocity would be protective of 

96% of species. It is possible that a lower intake threshold may be appropriate under 

certain conditions (e.g., Alaska’s limits to protect salmonids7); however, on a practical 

level, it is likely not appropriate to set a threshold lower than 0.5 fps on a national level.8

Some have argued that the data on intake velocity and fish swim speed support a higher 

threshold (up to 1.0 fps), but it should be recognized that intake screens rarely operate at 

optimal conditions. Given the potential for screen clogging and debris loading (which 

would reduce the open area of the screen and increase the through-screen velocity even 

further), the 0.5 fps threshold also provides for an appropriate safety margin for aquatic 

organisms.

Analyses were conducted to determine if the velocity threshold should vary by waterbody 

type. The swim speed data from the EPRI report was plotted by fish assemblage, a 

categorization of fish species by waterbody type (e.g., Pacific Ocean, rivers in the Eastern 

U.S., etc.). (See attached.) These plots did not show any clear differentiation of 

swimming ability between fish in the various waterbodies nor did any waterbody type 

appear to be any more vulnerable than another; it is therefore reasonable to conclude that 

the 0.5 fps national intake velocity limit is appropriate for all waterbody types.

 

6 According to data from EPA’s 2000 industry questionnaire, approximately 18% of intake structures meet 

the 0.5 fps threshold. Another 21% are less than 1.0 fps. See DCN 4-4023C “Preliminary Data Analyses 

Using Responses from the Detailed Industry Questionnaire: Phase II Cooling Water Intake Structures 

(Draft).”

7

See attached and DCNs 2-028A through D in the Phase I docket.

8 The incremental increase in organisms protected would not be well-balanced against the significant 

increase in costs for expanded screen area.
5

References

Alaska Department of Fish and Game (ADF&G). 1998. Water Intake Structures: an 

Alternative to Traditional Screened-Box Enclosures for the Protection of Fish. Prepared 

by Robert F. “Mac” Mclean for the ADF&G Habitat and Restoration Division. March 20, 

1998. Technical Report 97-8.

Electric Power Research Institute (EPRI). 2000. Technical Evaluation of the Utility of 

Intake Approach Velocity as an Indicator of Potential Adverse Environmental Impact 

under Clean Water Act Section 316(b). EPRI, Palo Alto, CA. 1000731.

Environmental Protection Agency (EPA). 2001. Preliminary Data Analyses Using 

Responses from the Detailed Industry Questionnaire: Phase II Cooling Water Intake 

Structures (Draft). DCN 4-4023C.